Quote:
Originally Posted by aerohead
Jim,if you can compare the area at the back of your new tail to the area of the back of the Insight,it will give you an idea of your new Cd.The profile drag varies directly with wake area.
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True, but we don't (and maybe can't) know the contributions of profile drag vs skin friction, nor how much of the profile drag happens at the rear of the vehicle.
Come to think of it, maybe you can estimate skin friction's contribution to Cd, by looking at the Cd of a flat plate in turbulent flow. That would give Cd=0.03 from skin friction, for any object that is roughly shaped like a compact car, assuming 100% turbulent (and perhaps also 0% detached) flow.
If a tail increases surface area by a third, you'd be looking at Cd=0.04 from skin drag, and the rest is form drag. But don't expect to reduce profile drag to zero by reducing transom area to zero. Basjoos achieved Cd=0.17 with a tail and other mods, and any car shaped just like GM's race prepped Impact with tailcone will have a Cd=0.153.
Darin, did you record before and after transom area and CdA on your Metro?